There is a need for tough, inexpensive thermoplastics that can be injection molded for use in applications such as household items. Styrene-butadiene copolymers and polyesters of terephthalic acid and ethylene glycol or copolyesters of terephthalic acid, ethylene glycol, and cyclohexanedimethanol meet most of these requirements, but have low impact strengths, especially at low temperatures. It has now been discovered that blends of styrene-butadiene copolymers with the above polyesters or copolyesters have unexpectedly high low-temperature impact strengths while maintaining other physical properties.
U.S. Pat. No. 4,582,876 dated Apr. 15, 1986 by D. A. Weemes and R. W. Seymour of Eastman Kodak describes blends of copolyesters and styrene-butadiene-maleic anhydride copolymers that have high impact at low temperatures. That disclosure did not make the present discovery obvious, however, since that styrenic polymer had rubbery inclusions that were believed to cause the high impact strengths of the blends. The morphology of the styrene-butadiene copolymer of the present invention is not similar to that disclosed in the '876 patent and was not expected to result in high impact strengths when blended with polyesters.
Also of interest are U.S. Pat. Nos. 4,117,034; 3,644,574; 4,352,907; 3,564,077; 4,096,202; 3,919,353; German Patent No. 3,332,325 and Japanese Patent No. 5371155.